1. Field of the Invention
Embodiments of the invention relate to analog-to-digital conversion, and, more specifically but not exclusively, to sampling-phase acquisition of analog-to-digital converters.
2. Description of the Related Art
In hard-disk drive (HDD) systems, a hard-disk platter is typically partitioned into concentric rings called tracks, and each track is further partitioned into smaller sections called sectors. Each sector typically stores a specified amount of user data (e.g., 512 bytes) and overhead information used by a read channel to recover the user data. For example, each sector may comprise, prior to the user data, a preamble that is used by the read channel to perform processing such as a zero-phase start and a zero-gain start, sampling-phase acquisition (also known as timing acquisition), and gain acquisition to lock on the user data stored on the corresponding sector. The zero-phase start and zero-gain start are performed to determine initial starting phase and gain, respectively, for the sampling-phase acquisition and gain acquisition, respectively. The preamble comprises a reference pattern known a priori to the read channel and is written each time that user data is written to a particular sector or a fragment of a sector.
When retrieving a sector of data from the hard-disk platter, a continuous-time analog waveform is obtained by passing a magneto-resistive read head over the hard-disk platter, and this readback waveform is sampled by the read channel using an analog-to-digital converter (ADC). The performance (e.g., Bit Error Rate (BER)) of the read channel in an HDD system is sensitive to the sampling phases at the ADC. Initially, the sampling phase at the ADC might not be a desired sampling phase (i.e., a sampling phase that ensures relatively good performance of the read channel); however, the sampling phase should be settled to a desired value before acquisition of the user data. Therefore, the read channel performs sampling-phase acquisition, wherein the sample phase (i.e., timing) is adjusted towards a desired sampling phase based on the preamble reference pattern known a priori to the read channel. The known reference pattern, and consequently the preamble, may be, for example, a 2T-pattern, which ensures that the corresponding analog readback waveform is a sinusoid with enough signal to noise ratio. The 2T-pattern refers to repetitions of a specific pattern, such as, a [1 1-1-1] pattern, which has a transition of −1 to 1 or 1 to −1 every 2T, where T is the time period allocated for every bit.